In the past, attempts have been made to provide devices to transduce the source to image distance (SID) for automatic X-ray radiography equipment. Even though the need for such devices has increased due to increasing automation of such equipment and increasing restrictions in government regulations, prior art SID monitors have exhibited less than ideal characteristics. One approach to meeting this need has been to provide a transducer utilizing a cable wound onto a grooved rewind drum or reel by means of a guide roller riding in the groove ahead of the cable. The linear motion to be measured is converted by the linear motion of the cable into angular or rotational motion of the drum which is then easily transduced by means of an electrical potentiometer having an output proportional to the angular displacement of its moveable element. In this type of SID monitor, a wide guide or slot was provided in the housing to allow the cable and guide roller to traverse across the grooved drum. Other SID monitors have not provided for level winding, that is winding at a constant diameter, and hence have failed to accurately convert linear into rotary motion, which resulted in an inaccurate source to image distance signal. Still other approaches to providing a SID monitor included running a pinion gear against a rack to translate linear into rotary motion which was then converted into an appropriate electrical signal. Such a gear system resulted in a complicated, costly apparatus which required careful alignment and which was subject to becoming jammed or broken in the event a foreign object was interposed between the pinion and rack.